1. Field of the Invention
The present invention relates generally to an apparatus and method for implementing a handoff in a CDMA communication system, and in particular, to an apparatus and method for controlling data communication during a handoff.
2. Description of the Related Art
The coverage area of a base station called a cell area is the radius of its cell in a mobile communication system. In a CDMA (Code Division Multiple Access) communication system, each cell is divided into three sectors, α, β, and γ. The service area of each sector is called a sector area. A cell area (or a sector area) can overlap with adjacent cell areas (or adjacent sector areas). The overlapped cell area (or sector area) is a cell boundary (or sector boundary) or a handoff area. In the CDMA communication system, each mobile station classifies base stations by sets according to the strengths of the signals received from the base stations. The system manages the base station sets and transmits set information to the mobile station. The sets include an active set, a candidate set, a neighbor set, and a remaining set. The definition and description of the sets are disclosed in the CDMA Specifications and only the active set related with a handoff will be described herein. A mobile station measures the reception power of a signal received from each base station (or sector) periodically and reports the measurement to the base station system. If the reception power is equal to or greater than a threshold, the base station system includes the base station in the active set of the mobile station and notifies the mobile station of the changed active set. An area where at least two base stations belong to the active set is a handoff area.
If a mobile station is located in a handoff area, it performs a handoff procedure according to the strengths of received signals. There are hard handoffs and soft handoffs. In the hard handoff, the mobile station discontinues communication with a serving base station and commences communication with a new base station. In the soft handoff, the mobile station commences communication with a new base station without interrupting communication with a serving base station. There are two kinds of soft handoffs: a handoff between two base stations and a softer handoff between sectors within a cell.
Unless otherwise noted, the term “sector” will cover both a cell and a sector hereinafter. Handoff will refer to soft handoff including softer handoff. Different communication schemes are employed in a handoff area according to service types in the existing CDMA communication systems. In CDMA systems mainly providing voice service, for example, IS-95A, IS-95B, and IS-2000 voice services, the mobile station transmits a voice traffic signal on the reverse link to all sectors belonging to the active set and receives voice traffic signals on the forward link. Therefore, the transmitted/received voice traffic signals are enhanced and the mobile station readily transitions to a different cell without interrupting voice service at a cell boundary where signal strength is relatively weak.
Meanwhile, for a data traffic service, the mobile station receives a data traffic signal from a sector offering the strongest reception power among the sectors of the active set and transmits a data traffic signal to all the sectors of the active set. The sector transmits a data traffic signal with transmission power higher than a voice traffic signal. In the case where the mobile station receives the same data traffic signal from a plurality of sectors, interference between the sectors deteriorates the detection performance of the data traffic signal. Consequently, the transmission rate of the data traffic signal receiving in the sectors is decreased. This problem is worse in an IS-2000 [HDR]1X EV DO (High Data Rate) system offering mainly data services.
A mobile station in a handoff area transmits a data traffic signal to all sectors of an active set as in the voice service.
As described above, a handoff occurs in the following way when an IS-2000 data service or an [HDR] 1X EV DO data traffic service is provided. A sector offering the strongest reception power among the sectors of an active set transmits a forward data traffic signal to a mobile station in a handoff area. The other sectors in the active set transmit forward data traffic signals to mobile stations other than the handoff mobile station. These forward data traffic signals interfere with the handoff mobile station. Because the interference is from the active set, it is more severe than interference from the other sets. Therefore, the interference signals from the other sectors in the active set adversely influence the traffic detection performance of the handoff mobile station.